Elsevier

Nitric Oxide

Volume 48, 1 August 2015, Pages 3-9
Nitric Oxide

Acute whole body UVA irradiation combined with nitrate ingestion enhances time trial performance in trained cyclists

https://doi.org/10.1016/j.niox.2014.09.158Get rights and content

Highlights

  • Ingestion of a nitrate gel increases plasma nitrate and nitrite concentration.

  • Plasma nitrite peaks 1.5 h after ingestion of a nitrate-rich gel.

  • Exposure to UV-A augments the increase in nitrite following the nitrate gel.

  • UV-A irradiation combined with dietary nitrate improves cycling performance.

  • Habitual sunlight exposure may influence outcomes of dietary nitrate studies.

Abstract

Dietary nitrate supplementation has been shown to increase nitric oxide (NO) metabolites, reduce blood pressure (BP) and enhance exercise performance. Acute exposure to ultraviolet (UV)-A light also increases NO bioavailability and reduces BP. We conducted a randomized, counterbalanced placebo-controlled trial to determine the effects of UV-A light alone and in combination with nitrate on the responses to sub-maximal steady-state exercise and time trial (TT) performance. Nine cyclists (VO2max 53.1 ± 4.4 ml/kg/min) completed five performance trials comprising 10 min submaximal steady-state cycling followed by a 16.1 km TT. Following a familiarization the final four trials were preceded, in random order, by either (1) Nitrate gels (NIT) + UV-A, (2) Placebo (PLA) + UV-A, (3) NIT + Sham light (SHAM) and (4) PLA + SHAM (control). The NIT gels (2 × 60 ml gels, ~8.1 mmol nitrate) or a low-nitrate PLA were ingested 2.5 h prior to the trial. The light exposure consisted of 20 J/cm2 whole body irradiation with either UV-A or SHAM light. Plasma nitrite was measured pre- and post-irradiation and VO2 was measured continuously during steady-state exercise. Plasma nitrite was higher for NIT + SHAM (geometric mean (95% CI), 332 (292–377) nM; P = 0.029) and NIT + UV-A (456 (312–666) nM; P = 0.014) compared to PLA + SHAM (215 (167–277) nM). Differences between PLA + SHAM and PLA + UV-A (282 (248–356) nM) were small and non-significant. During steady-state exercise VO2 was reduced following NIT + UVA (P = 0.034) and tended to be lower in NIT + SHAM (P = 0.086) but not PLA + UV-A (P = 0.381) compared to PLA + SHAM. Performance in the TT was significantly faster following NIT + UV-A (mean ± SD 1447 ± 41 s P = 0.005; d = 0.47), but not PLA + UV-A (1450 ± 40 s; d = 0.41) or NIT + SHAM (1455 ± 47 s; d = 0.28) compared to PLA + SHAM (1469 ± 52 s). These findings demonstrate that exposure to UV-A light alone does not alter the physiological responses to exercise or improve performance in a laboratory setting. A combination of UV-A and NIT, however, does improve cycling TT performance in this environment, which may be due to a larger increase in NO availability.

Introduction

Since Larsen and colleagues [1] first reported a reduced oxygen cost of exercise following ingestion of sodium nitrate, a growing number of studies have demonstrated the ergogenic effects of dietary nitrate supplementation on athletic performers [2], [3], [4], [5]. Research has shown that ingestion of nitrate-rich food such as beetroot and spinach can increase circulating levels of nitric oxide (NO) metabolites via an NO synthase (NOS) independent pathway [6]. Following absorption of nitrate from the stomach into the plasma, nitrate is transported into the saliva via the salivary glands. Bacteria then reduce the nitrate to nitrite [7] where, following ingestion, nitrite is potentially reduced to NO when exposed to hypoxic [8] or acidic environments [9].

This increase in NO related products, potentially leading to an increase in NO production, has been typically shown to reduce oxygen consumption (VO2) during sub-maximal steady-state exercise [1], [10] and improve both exercise capacity and performance [2], [3], [5], [10]. Conversely, several other studies report exercise performance of varying modalities to be unaltered by dietary nitrate supplementation [11], [12], [13]. While there are various methodological differences between studies that may account for this disparity, it has become apparent that ergogenic effects appear minimal in highly-trained endurance cohorts [11], [14]. This may be partly explained by the higher baseline nitrate/nitrite pool in endurance trained athletes compared to untrained matched controls [15]. Evidence from murine models also suggests that increases in muscle blood flow and contractile force production following nitrate supplementation only occur in Type II muscle fibers [16], [17]. One may, therefore, reasonably assume that elite endurance athletes, who are known to have high proportions of type I muscle fibers, would be less likely to benefit from nitrate supplementation [18]. Alternatively, using only a single low dose of dietary nitrate (~4–5 mmol) may explain the resultant diminished ergogenic effect during exercise [12], [13], [19].

Intriguingly, exposing the skin to the ultra violet (UV)-A component of sunlight increases circulating plasma nitrite via decomposition of photo reactive nitrogen oxides stored in dermal cells [20], [21]. Opländer and colleagues [21] observed that systemic blood pressure (BP) was reduced by 11% 30 min after UV-A exposure, a finding similar to that of dietary nitrate [22]. More recently, Liu and colleagues [23] provided some mechanistic basis for these findings by demonstrating UV-A induced NO production also increases forearm blood flow. Therefore, given that exposure to UV-A light increases NO bioavailability sufficiently to induce measureable physiological effects; it is plausible that it may also enhance exercise performance. Furthermore, it remains to be determined whether combining UV-A exposure with concomitant nitrate supplementation may potentiate a synergistic effect on NO bioavailability, given that both offer different routes for increasing plasma NO related products.

Despite this, the effects of UV-A exposure, either alone or in combination with nitrate supplementation, on plasma nitrite and parameters of exercise performance are currently unknown. This is of interest given the reported benefits to exercise performance associated with an increased availability of NO related products [2], [3], [5], [10]. Therefore, the aim of this study was to determine the effects of acute UV-A light exposure with and without simultaneous nitrate supplementation on plasma nitrite, the physiological responses to steady-state exercise and cycling time trial (TT) performance. We hypothesized that: (1) UV-A exposure would increase plasma nitrite and improve exercise performance and (2) UV-A exposure combined with nitrate would coalesce to increase plasma nitrite and improve exercise performance to a greater extent than either intervention alone.

Section snippets

Participants

Nine male trained-cyclists and triathletes (age 36 ± 6 years, stature 182 ± 5 cm, body mass 78.9 ± 6.0 kg, and VO2max 53.1 ± 4.4 mL⋅kg–1⋅min–1) volunteered and provided written informed consent to participate in the study that was approved by the School of Science Ethics Committee at The University of the West of Scotland. All participants were amateur competitive athletes who typically completed a minimum of two cycling training sessions per week and regularly competed in TT competitions. All

Plasma nitrite responses during intervention study

Plasma nitrite data from the intervention study is presented in Fig. 1. There was a significant main effect of ‘condition’ (P= 0.001) on plasma nitrite concentration. There was no significant main effect for time (P= 0.944) or a condition × time interaction (P = 0.083). Prior to the light-exposure, plasma nitrite in the NIT + SHAM (399 (345–461) nM) condition was higher than in the control (247 (179–343) nM, P= 0.024, 95% CI 17–257 nM). Plasma nitrite in the NIT + UV-A (391 (291–526) nM))

Discussion

Exogenous supplementation with dietary nitrate increases the bioavailability of NO which has been shown in some conditions to reduce the oxygen cost of exercise and improve performance [2], [3], [4], [5]. The present study explored the physiological and ergogenic effects of short-term exposure to UV-A light as a novel method to increase circulating NO metabolites both with and without ingestion of NIT. The principal finding was that exposure to UV-A light alone was not sufficient to

Conclusion

The principal findings of the present study were that exposure to UV-A light subsequent to ingestion of a NIT improved the physiological responses to steady-state exercise and 16.1 km cycling TT performance. Furthermore, we provide some evidence of a cumulative effect of dietary nitrate and UV-A derived NO, whereby the increase in plasma nitrite was larger than with either intervention alone. This study offers the intriguing possibility that a combination of naturally occurring environmental

Acknowledgments

The authors wish to thank Science in Sport who provided the NIT and PLA supplements for this study free of charge.

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